U.S. President Donald Trump threatened former Russian leader Dmitry Medvedev — and dismissed him as a “failed president.”

“Russia and the USA do almost no business together. Let’s keep it that way, and tell Medvedev, the failed former President of Russia, who thinks he’s still President, to watch his words. He’s entering very dangerous territory!” Trump wrote in the early hours of Thursday morning.

Medvedev, another social media motormouth, has ridiculed Trump’s ultimatum to the Kremlin, in which the U.S. president shortened his deadline for the Kremlin to end the war in Ukraine or face crippling economic consequences.

“Trump’s playing the ultimatum game with Russia: 50 days or 10 … He should remember 2 things: 1. Russia isn’t Israel or even Iran. 2. Each new ultimatum is a threat and a step towards war,” Medvedev blasted on social media earlier this week. “Not between Russia and Ukraine, but with his own country. Don’t go down the Sleepy Joe road!”

In another post on X, Medvedev, who is currently the deputy chair of Russia’s security council, called U.S. Senator Lindsey Graham “gramps,” after the Republican foreign policy hawk told him to “get to the peace table.”

“It’s not for you or Trump to dictate when to ‘get at the peace table’. Negotiations will end when all the objectives of our military operation have been achieved. Work on America first, gramps!” Medvedev fumed.

Trump in July originally set a 50-day deadline for Russia to reach a ceasefire with Ukraine, threatening tariffs if a deal was not made.

Medvedev was Vladimir Putin’s prime minister, before swapping places with him and serving as a placeholder president from 2008-2012 because Russian law barred Putin from a third consecutive term. Medvedev is a vocal supporter of Putin’s full-scale invasion of Ukraine and frequent social media attack dog for the Kremlin.